Ochratoxin production by Aspergillus ochraceus as affected by methionine and structurally related compounds

When 10^-2 M of L- or D-methionine was added to a synthetic medium containing xylose as the sole carbon source, ochratoxin production by A. ochraceus was strongly inhibited. At that concentration methionine derivatives, e.g., α-methyl-DL-methionine, DL-methionine sulfoxide, and L-methionine sulfone, did not inhibit ochratoxin production, whereas DL-methionine S-methyl sulfonium chloride (MMSC) inhibited ochratoxin production to a large extent. L-Methionine, as well as MMSC, also completely inhibited sclerotia formation, while D-methionine and DL-methionine sulfoxide caused only a partial inhibition. At lower concentrations (10^-3 and 10^-4 M), none of the compounds exhibited inhibitory effects. In cases where strong ochratoxin inhibition was detected, fungal radial growth or mycelial dry weight was inhibited by only 10-25%, while the initial pH of the medium dropped from ~6.5 to ~4.4-5.0. Adjustment of the initial pH of media supplemented with 10^-2 M L-methionine, D-methionine, or MMSC to a pH of ~7.8 did not change the inhibitory effects on ochratoxin production in media containing L-methionine. On the other hand, sclerotia formation was restored in all three treatments.